1. Field of the Invention
The invention relates to a neutral-colored soda-lime-silicate glass with a basic glass composition of 66-75 weight % SiO.sub.2, 10-20 weight % Na.sub.2 O, 5-15 weight % CaO, 0-5 weight % MgO, 0-5 weight % Al.sub.2 O.sub.3, 0-5 weight % K.sub.2 O, as well as colorants Fe.sub.2 O.sub.3 (total iron content) and cobalt.
Neutral-colored in connection with the invention means that the glass does not have any significant coloration in respect of transmission, and thus can be termed clear transparent. In technical circles, such glass is termed low-iron or white glass.
2. Description of the Prior Art
It is a known phenomenon that with such neutral-colored, that is to say, clear transparent, glass in respect of transmission, the edges of the glass sheets produced from it appear nevertheless to be colored. Thus, one observes with such glass, irrespective of the sheet thickness, a more or less intense green tinge of the glass edges, which is attributable to practically unavoidable impurities, such as the presence of iron particles, in the raw materials used. Even if colored glass edges are acceptable for most of the applications of the glasses, there are frequently aesthetic problems with the green hue, particularly when the glass is arranged directly adjacent to objects of different colors, for example in articles of furniture, or when the glass sheets, such as for example table tops with visible glass edges, are arranged such that their edges are in the direct field of view of the observer.
For formation of glass edges appearing azure, EP 0 463 607 B1 suggests, in addition to minimizing the iron content in the glass to less than 0.02 weight % Fe.sub.2 O.sub.3 (total iron content), that the redox ratio, that is to say the ratio of the iron present in bivalent state (FeO, expressed as Fe.sub.2 O.sub.3) to the total iron content (expressed as Fe.sub.2 O.sub.3) should be adjusted to at least 0.44. The high content of bivalent iron has the effect that the glass edges assume an azure tinge, the transparency of this glass remaining of course unaffected thereby.
In conventional flat glass furnaces and under the melting conditions generally customary for the production of sulfate-refined soda-lime-silicate glass, it is not possible to achieve such high redox ratios. The redox ratios achievable with such melting systems without special measures are in the range of between 0.15 and 0.35. Higher redox ratios can only be achieved in conventional glass furnaces by using special, very costly additional glass raw materials and with suitable modification of the melting process. Another, equally costly option is the use of special glass furnaces.
In addition, work has been carried out with other colorant additives in order to obtain the most widely varying range of tints of the edges of a glass sheet. Thus, EP 0 463 606 B1 teaches, with low iron content of less than 0.02 weight % Fe.sub.2 O.sub.3 (total iron content), that selenium be added in the amount of 3.multidot.10.sup.-5 weight % to 2.multidot.10.sup.-4 weight % in order to obtain honey or wood-colored edges of a glass sheet. For the purpose of tinting, cobalt in the form of cobalt oxide (CoO) is added if necessary, that is to say in the maximum amount of 3.multidot.10.sup.-4 weight %. For production of the glass described in EP 0 463 606 B1, a special glass furnace is recommended.